I. Field of the Invention
This invention relates generally to electronic switching means, and more particularly to an improved microstrip shunt-mounted PIN diode switch for use at millimeter wave frequencies and having a DC biasing arrangement which obviates the need for blocking capacitors for isolating the DC bias voltage from the RF circuitry with which the switch is intended to be used.
II. Discussion of the Prior Art
In high frequency work, so-called microstrip technology is used in implementing a wide number of electronic circuit devices. Such a microstrip typically comprises an insulating substrate having a conductive ground plane adhered to one surface thereof and an etched conductive pattern on the other major surface. By properly following the design rules which associate operating frequencies with geometries, the etched pattern can be used to create electronic circuits including distributed capacitors and inductors whose impedance values at the operating frequencies can be tailored to meet desired circuit requirements. The etched microstrip patterns may also be utilized in combination with active components, e.g., Gunn diodes, IMPATT diodes, etc., to yield high frequency oscillators having sufficient power to drive antennas in radar systems and the like. Because of their physical makeup, microstrip circuits tend to remain quite stable and are often employed where size constraints will not permit other technologies employing discrete components to be used.
In certain radar applications, a need exists for switching an RF source between a plurality of antennas and, alternatively, coupling a received signal from plural antennas to a single receiver. There is disclosed in the prior art, a millimeter wave shunt-mounted PIN diode switch in which the DC ground for the bias sources and the RF ground are common. In this design, the DC ground and the RF ground are both the ground plane of the microstrip device. It then becomes necessary to include a number of DC blocking capacitors in circuit with the microstrip to prevent the DC bias current used to switch the PIN diodes from flowing into the sources or loads and to isolate the biases of each diode from one another.
Referring to FIG. 1, which shows a prior art arrangement of a millimeter wave shunt-mounted PIN diode switch designed to couple an RF input to one or the other of a pair of outputs, first and second bias supplies are used to control the on/off state of the PIN diodes. When a diode is forward biased, it presents a low loss to RF energy but when reversed biased affords a high impedance. Thus, for example, if the DC biases no. 1 and no. 2 are such that the PIN diode no. 1 is forward biased, while PIN diode no. 2 is reverse biased, the RF output will appear at output no. 2 because output no. 1 is effectively held at RF ground potential. On the other hand, if the bias is such that it is diode no. 2 that is forward biased and diode no. 1 that is reverse biased, then the RF input will be switched to output no. 1.
It is to be especially noted relative to the prior art circuit of FIG. 1 that four blocking capacitors C.sub.1 through C.sub.4 are required to block the DC bias current from flowing back into the input source, the output loads and for preventing the DC bias from source no. 1 from reaching PIN diode no. 2 or the bias from DC source no. 2 from reaching the PIN diode no. 1. Because the DC voltage required to bias the diodes is conducted by the microstrip transmission lines, to isolate the bias voltages from one another, the DC blocking capacitors are incorporated and are designed to pass the RF frequency signals while inhibiting the DC bias current flow. Those skilled in the art will appreciate, however, that the presence of these blocking capacitors tends to add additional loss to the performance of the switch. In addition, because they are high-frequency components, they tend to be very small and somewhat expensive, thus adding to the material and labor costs of fabricating the switch. Moreover, in certain microminiaturized electronic packaging applications, the inclusion of discrete blocking capacitors is prohibited because of the space constraints imposed. Moreover, the circuit incorporating the discrete component blocking capacitors may exhibit decreased reliability, especially under high G forces or vibration.